The present invention relates to an apparatus for use in a system for producing a composite video signal for broadcasting by switching or composing a plurality of composite video signals supplied from a plurality of television cameras, for adjusting automatically phases of the plurality of composite video signals.
In the system for effecting a television broadcasting by means of switching methods such as a cut-switching, mixing etc. for the composite video signals supplied from a plurality of television cameras to a video switcher use is generally made of a video mixing amplifier. In this case the most important condition is that composite synchronizing signals and burst flag signals of a plurality of the composite video signals supplied from a plurality of television cameras should have identical phases at a cross point in the video mixing amplifier at which the cut-switching is effected. If this condition is not satisfied, it is sometimes impossible to construct or reproduce a usefull television picture. In the television cameras with synchronizing generator locked circuits, a black burst signal including a composite synchronizing signal and a burst flag signal is derived from a reference synchronizing signal generator provided in the video mixing amplifier and the composite video signals are transmitted in synchronism with the thus derived black burst signal. However, in practice the composite video signals from a plurality of television cameras have usually different phases at the cross point sue to the fact that the cameras operate under various conditions. In order to overcome such a drawback, in practice lengths of co-axial cables connecting the video mixing amplifier and the respective cameras are adjusted or a horizontal synchronizing phase shifting circuit and a subcarrier signal phase shifting circuit provided in the synchronizing generator locked circuit are adjusted. In this phase adjusting method a relatively long time is required to set the video and mixing system including the television cameras into a usefull condition for broadcasting. Thus the promptitude of collection of subject matter for the television broadcasting could not be expected. Further there is another drawback that required properties could not be always guaranteed due to secular change of components and temperature variation.